An image forming apparatus reads an image from an original document, forms a toner image corresponding to the read image on a sheet, and fixes the toner image on the sheet using a fixing device.
The fixing device holds the sheet between a rotating member, for example, a fixing roller or a heat belt and a pressing member, for example, a press roller and applies heat and pressure to the sheet to thereby fix the toner image on the sheet.
An excitation coil for induction heating is provided near the rotating member. High-frequency current flows to the excitation coil, whereby a high-frequency magnetic field is generated from the excitation coil. Eddy-current is generated in the rotating member by the high-frequency magnetic field. The rotating member generates heat with Joule heat based on the eddy-current.
When a small-size sheet passes between the rotating member and the pressing member, the temperature on both the sides of the rotating member not in contact with the sheet is higher than the temperature in the center of the rotating member in contact with the sheet. When the temperature on both the sides of the rotating member rises to be higher than the temperature in the center thereof, the hardness of an elastic member such as rubber forming both the sides of the rotating member falls earlier than the hardness of an elastic member such as rubber forming the center. Therefore, the life of the rotating member is reduced.
Immediately after the small-size sheet passes between the rotating member and the pressing member, when a full-size sheet passes between the rotating member and the pressing member, a toner on the full-size sheet offsets to both the sides of the rotating member in a high-temperature state and causes a fixing failure.
Therefore, in JP-A-2001-318545, first cores are arranged in a position corresponding to an area where sheets of all usable sizes pass and second cores having a Curie point lower than that of the first cores are arranged in positions corresponding to areas excluding the passing area. When temperature rises to exceed the Curie point, the magnetic permeability of the second cores falls and the second cores prevent magnetic fluxes from passing. Consequently, a temperature rise of the rotating member in the sheet non-passing area is suppressed.
However, in this case, since two kinds of cores having different Curie points have to be prepared, cost increases.